First Report of Phytophthora megasperma Associated with Decline and Death of Common Walnut Trees in Italy

Abstract

Common walnut (Juglans regia L.) is an important nut crop in Italy, which is the fifth largest producer of walnut in Europe. In recent years, walnut decline and subsequent death has increased in many Italian commercial orchards. In the summer of 2010, several declining trees were present in waterlogged area of a walnut orchard located in the Veneto region. Symptoms included sparse foliage, wilting, and shoot and branch dieback. By the next year, a larger area of about 1 ha with 20% of dead trees was present, and soil/root samples were subjected to azalea leaf baiting and successively cultured on PARBH medium (3). Isolates were identified as Phytophthora megasperma based on morphological characteristics (2) and DNA sequence analysis. Sporangia were 35.0 to 62.0 × 12.0 to 30.0 μm, nonpapillate, and noncaducous when produced in soil extract solution. Oogonia had an average diameter of 36 μm with mostly paragynous antheridia. Identity was confirmed by sequence comparison in NCBI database with 99% and 100% identity for internal transcribed spacer (ITS) and mitochondrial partial COI (4) for cytochrome oxidase subunit 1, respectively. The sequences of the isolate AB199 were deposited in GenBank with the accession nos. HE805270 and HE805269 for ITS and COI, respectively. Pathogenicity tests were conducted in the greenhouse on six 1-year-old walnut shoots with two inoculation points each. Mycelial plugs cut from the margins of actively growing 8-day-old cultures on PDA were inserted through the epidermis to the phloem. Controls were treated as described above except for inoculation with sterile PDA plugs. After inoculation, shoots were incubated in test tubes with sterile water for 1 week in the dark at 22 ± 2°C. Lesions were evident at the inoculation points. P. megasperma was consistently reisolated from the margin of symptomatic tissues. Controls remained symptomless. P. megasperma is a polyphagous, ubiquitarious Phytophthora species that attacks many crops and fruit species including walnut. Although several other species of Phytophthora have been reported from Italy (1), to the best of our knowledge, this is the first report of Phytophthora decline on common walnut in Italy caused by P. megasperma. References: (1) A. Belisario et al. Acta Hort. 705:401, 2006. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (3) G. C. Papavisas et al. Phytopathology 71:129, 1981. (4) G. P. Robideau et al. Mol. Ecol. Resour. 11:1002, 2011.